1. Field of the Invention
The present invention relates to a control system for an automobile automatic transmission, and, more particularly, to an automatic transmission control system whose hydraulic circuit has an accumulator for activating a frictional coupling element in a reverse range of an automatic transmission.
2. Description of Related Art
Typically, automatic transmissions for automobiles have a torque converter and a transmission mechanism which includes a plurality of frictional coupling elements selectively coupled and uncoupled to place the automatic transmission into any one of desired transmission modes and gear ranges according to driving conditions. Selectively coupling and uncoupling of these frictional coupling elements is performed by the use of hydraulically operated actuators. When shifting such an automatic transmission into a reverse (R) range from a neutral (N) range, a reverse gear is provided by locking a specific one of the frictional coupling elements, which has been unlocked or released in the neutral (N).
The automatic transmission transmits the engine output torque multiplied at a large ratio corresponding to a reduction ratio of the reverse gear to wheels when locking the specific frictional element to enable reverse gear during a shift from the neutral range to the reverse range (which is hereafter referred to as an N-R shift), which can produce shift shocks which are so-called N-R shift shocks and give a driver an uncomfortable feeling of gear shift during an N-R shift.
In order for the automatic transmission to eliminate such an N-R shift shock, it is typical to provide an accumulator in a hydraulic circuit for generating hydraulic pressure changing in level by way of what is called "shelf pressure" and activating or locking the specific frictional coupling element with the shelf pressure. The term "shelf pressure" as used in this specification shall mean and refer to transitional pressure increasingly or decreasingly changing at an incline or gradient more gentle or sluggish before and after the change.
In some automatic transmissions of this type, due to structural or mechanical considerations, the transmission gear mechanism necessarily includes two frictional coupling elements which are simultaneously locked to provide a reverse gear. Such an automatic transmission produces considerably heavy shift shocks during an N-R shift when the two frictional coupling elements are locked. In an attempt to reduce N-R shift shocks, these two frictional coupling elements are designed and adapted to be locked with a time delay relative to each other by means of the operation of an accumulator provided in a hydraulic circuit of either one of them. An important factor in the designing of the automatic transmission control system is the selection of which of these frictional coupling elements shall have its hydraulic circuit so controlled regarding the accumulator.
This is because some frictional coupling elements, such as clutches and brakes, cooperate with hydraulic circuits relating thereto which are provided with what is called drift-on-balls for canceling centrifugal hydraulic force exerting on the frictional coupling elements which are generally different in operational characteristics from one another. Because operation and fabrication of drift-on-balls is well known to those skilled in the art of hydraulic control, only a brief description will be hereafter given. In a hydraulic control system a frictional coupling element is fixedly mounted for rotation on and placed remote from a rotary shaft. Hydraulic pressure or working oil is introduced into an axially extending passage in the rotary shaft and then into a radial passage so as to be supplied to the frictional coupling element. During rotation of the shaft, the working oil remaining in the passage tends to act on the frictional coupling element due to centrifugal force. With an increase in centrifugal force, the frictional coupling element is adversely affected by the working oil. However, the drift-on-ball installed in the passage moves axially with an increase in centrifugal force and shuts down the communication of the passage with the frictional coupling element. In addition, some clutches and brakes have diaphragm springs used as return springs which are generally different in operational characteristics from one another. For these reasons, when the accumulator is provided in connection with one of the frictional coupling elements which and/or whose hydraulic circuit have differences in operational characteristics from the remaining frictional coupling elements, the accumulator must generate hydraulic pressure including shelf pressure with a large incline or gradient so as to absorb or cancel its peculiar characteristic operational differences. However, this is unfavorable for the demands of avoidance of shift shocks.
Accordingly, in the automatic transmission having two specific frictional coupling elements, such as a reverse clutch and a low/reverse brake, which are simultaneously locked to provide a reverse gear, the accumulator is preferably provided in a hydraulic circuit in connection with the low/reverse brake which has operational characteristics less different from other low/reverse brakes. With the provision of the accumulator in a hydraulic circuit in connection with the low/reverse brake, the automatic transmission performs locking of the low/reverse brake with a time delay from locking of the reverse clutch during a gear shift to the reverse gear, so as to reduce effectively N-R shift shocks. On the other hand, the automatic transmission encounters shift shocks during an R-N shift resulting from performance of unlocking of the low/reverse brake with a time delay from unlocking of the reverse clutch. This shift shock is caused due to an abrupt torque change during the unlocking of the reverse clutch and gives a driver an uncomfortable feeling of gear shift.
In an attempt to reduce shift shocks generated in the automatic transmission of this type when both reverse clutch and low/reverse brake are locked to provide a reverse gear, it is desirable to provide an accumulator and a shift valve in a low/reverse brake activating circuit in order from the upstream side, but the downstream side of a reverse clutch activating circuit, so that the shift valve changes the communication of a pressure line from the low-reverse brake to a drain line upon a shift of the automatic transmission from the reverse (R) range to any possible ranges other than the reverse (R) ranges. Such an automatic transmission control system is known from, for instance, Japanese Unexamined Patent Publication No. 4-248,064.
With the automatic transmission described in the above publication, since the accumulator causes the low/reverse brake to be locked with a time delay from locking of the reverse clutch during an N-R shift, N-R shift shocks are considerably reduced. Conversely, during an R-N shift, while the shift valve changes the communication of the pressure line from the low/reverse brake to the drain line, the accumulator, disposed upstream from the shift valve in the hydraulic circuit relating to the reverse clutch, is brought into communication with the hydraulic circuit relating to the reverse clutch and unlocking of the reverse clutch is caused with a time delay from unlocking of the low/reverse brake. As a result, shift shocks are effectively reduced.
However, the automatic transmission control system described in the above publication encounters inter-lock of the transmission gear mechanism which possibly causes an engine stall or engine stop. For example, during a shift from the reverse (R) range to the drive (D) range (which is hereafter referred to as a R-D shift) in which both low/reverse brake and reverse clutch are unlocked, because the accumulator is brought into communication with the hydraulic circuit for activating the reverse clutch, unlocking activation of the reverse clutch is caused with a delay. On the other hand, when the automatic transmission is in the drive (D) range, locking of a forward clutch, which is one of the frictional coupling elements activated to provide forward gears, starts when line pressure is supplied. In this instance, when locking of the forward clutch is accomplished prior to the completion of unlocking of the reverse clutch, the transmission gear mechanism falls into an inter-lock, keeping the frictional coupling elements slipping for a long time. This causes a deterioration in the durability of the frictional coupling elements and, if the worst happens, causes an engine stop or engine stall. It may be of course effective to install a diaphragm and an accumulator in the hydraulic circuit for the forward clutch so as to provide a delay of locking of the forward clutch. However, since, when taking the responsiveness of the forward clutch during an N-D shift into account, there is marginal time in the delay of locking, there still remains the problem of engine stalls.
The same problem possibly occurs during a shift from the drive (D) range to the reverse (R) range (which is hereafter referred to as a D-R shift). Specifically, in the event of locking of the reverse clutch accomplished prior to the completion of unlocking of the forward clutch, the transmission gear mechanism falls into an inter-lock, resulting in an engine stop or engine stall.